1. Field of the Invention
The present invention relates to systems for magnetically-assisted abrasive finishing and polishing of substrates; more particularly, to such systems employing magnetorheological (MR) polishing fluids; and most particularly, to an improved and low-cost system wherein polishing operation does not require an MR fluid delivery system and is carried out by a magnetically stiffen polishing ribbon formed by a novel integrated fluid management module (IFMM) charged with MR polishing fluid and having sensors and MR fluid conditioning devices to provide appropriate dynamic control of MR fluid properties.
2. Background of the Invention
Use of magnetically-stiffened magnetorheological fluids for abrasive finishing and polishing of substrates is well known. Such fluids, containing magnetically-soft abrasive particles dispersed in a liquid carrier, exhibit magnetically-induced thixotropic behavior in the presence of a magnetic field. The apparent viscosity of the fluid can be magnetically increased by many orders of magnitude, such that the consistency of the fluid changes from being nearly watery to being a very stiff paste. When such a paste is directed appropriately against a substrate surface to be shaped or polished, for example, an optical element, a very high level of finishing quality, accuracy, and control can be achieved.
U.S. Pat. Nos. 5,449,313 issued Sep. 12, 1995 and 5,577,948 issued Nov. 26, 1996, both to Kordonsky et al. disclose magnetorheological polishing devices and methods.
U.S. Pat. No. 5,525,249 issued Jun. 11, 1996 to Kordonsky et al. discloses magnetorheological fluids and methods of making thereof.
U.S. Pat. Nos. 5,839,944 issued Nov. 24, 1998 and 6,106,380 issued Aug. 22, 2000, both to Jacobs et al. disclose methods and apparatus for deterministic magnetorheological finishing of substrates.
U.S. Pat. No. 5,951,369 issued Sep. 14, 1999 to Kordonski et al., the disclosure of which is hereby incorporated by reference, discloses a system for deterministic magnetorheological finishing of substrates. This patent is referred to herein as “'369.”
In an exemplary MR polishing interface, a convex lens (also referred to herein as a “workpiece”) to be polished is installed at some fixed distance from a moving wall, so that the lens surface and the wall form a converging gap. Typically, the lens is mounted for rotation about an axis thereof. An electromagnet, placed below the moving wall, generates a non-uniform magnetic field in the vicinity of the gap. The magnetic field gradient is normal to the wall. The MR polishing fluid is delivered to the moving wall just above the electromagnet pole pieces to form a polishing ribbon. As the ribbon moves in the field, it acquires plastic Bingham properties and the top layer of the ribbon is saturated with abrasive due to levitation of non-magnetic abrasive particles in response to the magnetic field gradient. Thereafter, the ribbon, which is pressed against the wall by the magnetic field gradient, is dragged through the gap resulting in material removal from the lens in the lens contact zone. This area is designated as the “polishing spot” or “work zone”. The rate of material removal in the polishing spot can be controlled by controlling the strength of the magnetic field, the geometrical parameters of the interface, and the wall velocity.
The polishing process employs a computer program to determine a CNC machine schedule for varying the velocity (dwell time) and the position of the rotating workpiece through the polishing spot. Because of its conformability and subaperture nature, this polishing tool may finish complex surface shapes like aspheres having constantly changing local curvature.
A fundamental advantage of MRF over competing technologies is that the polishing tool does not wear, since the recirculating fluid is continuously monitored and maintained. Polishing debris and heat are continuously removed. The technique requires no dedicated tooling or special setup. Integral components of the MRF process are the MRF software, the CNC platform with programmable logic control, the MR fluid delivery and recirculating/conditioning system, and the magnetic unit with incorporated carrier surface. The carrier surface can be formed, for example, by the rim of a rotating wheel, by horizontal surface of a rotating disk, or by a continuous moving belt.
In a typical prior art magnetorheological finishing system, such as is disclosed in '369, a carrier surface is formed on a vertically-oriented non-magnetic wheel having an axially-wide rim which is undercut symmetrically about a hub. Specially-shaped magnetic pole pieces, which are symmetrical about a vertical plane containing the axis of rotation of the wheel, are extended toward opposite sides of the wheel under the undercut rim to provide a magnetic work zone on the surface of the wheel, preferably at about the top-dead-center position. The carrier surface of the wheel may be flat, i.e., a cylindrical section, or it may be convex, i.e., a spherical equatorial section, or it may be concave. The convex shape can be particularly useful as it permits finishing of concave surfaces having a radius longer than the radius of the wheel.
Mounted above the work zone is a workpiece receiver, such as a chuck, for extending a workpiece to be finished into the work zone. The chuck is programmably manipulable in a plurality of modes of motion and is preferably controlled by a programmable controller or a computer.
Magnetorheological polishing fluid, having a predetermined concentration of non-magnetic abrasive particles and magnetic particles which are magnetically soft, is extruded in a non-magnetized state, typically from a shaping nozzle, as a ribbon onto the work surface of the wheel, which carries it into the work zone where it becomes magnetized to a pasty consistency. In the work zone, the pasty MR polishing fluid does abrasive work on the substrate. The exposure of the MR fluid to air causes some evaporation of carrier fluid and a consequent concentrating of the MR fluid. Exiting the work zone, the concentrated fluid becomes non-magnetized again and is scraped from the wheel work surface for recirculation and reuse.
Fluid delivery to, and recovery from, the wheel is managed by a closed fluid delivery system as disclosed in U.S. Pat. No. ‘369’ or by an improved system as disclosed in U.S. Pat. No. 6,955,589. MR fluid is withdrawn from the scraper by a suction pump and sent to a delivery pump tank where its temperature is measured and adjusted to aim. Recirculation from the delivery pump to the nozzle, and hence through the work zone, at a specified flow rate is accomplished by controlling the delivery pump flow rate through the use of a magnetic valve, the hydraulic resistance being controlled by feed-back signal from a flow meter.
The concentration of solids in the MR fluid as discharged onto the wheel is an important factor in controlling the rate of material removal in the work zone. Concentration control is accomplished by measurements and monitoring of fluid viscosity which correlates directly with concentration. Viscosity measurements are carried out by an in-line capillary viscometer. At a constant fluid flow rate, the pressure drop through the capillary tubing, that is, the pressure difference between the two pressure sensors, is proportional to the viscosity of the fluid. An increase in pressure drop is inferred to mean an increase in viscosity and is used to cause replenishment of carrier fluid into the MR fluid in the tempering pump tank to reduce the apparent viscosity to aim.
Several problems have been encountered in using the U.S. Pat. Nos. '369 and '589 disclosures to finish substrates.
Operation of the prior art MR finishing system requires use of a delivery system which comprises a delivery pump, a suction pump, a flow meter, a viscometer, a nozzle, pressure transducers, a pulse dampener, a magnetic valve, a chiller, and tubing. Cost of such a delivery system is significant and may constitute up to quarter of the total cost of the MR finishing system.
Recharging of the delivery system is a time-consuming process, requiring complete disassembling, cleaning of all components, re-assembly, and breaking in after charging with a fresh fluid, which lengthy procedure negatively affects productivity and flexibility of technology.
The delivery system must operate in a non-stop regime during the MR fluid's “life” in the machine. Continuous recirculation of abrasive MR fluid is required even in the intervening periods between polishing in order to avoid changes in MR fluid properties due to sedimentation of solids. Such continuous recirculation results in accelerated wear and tear of delivery system components and consumption of extra energy.
MR fluid flow rate instability (pulsations) in the delivery system due to any of several causes results in unstable removal rate and errors on the substrate surface.
To provide proper circulation of MR fluid and compatibility with different components of the delivery system, the fluid must have specific rheological/viscous properties and appropriate chemistry. This limits selection of fluid components and restricts fluid composition, for example, for greater solids concentration required for enhancement of the removal rate.
What is needed in the art is an improved, low cost, low maintenance and technologically flexible MR finishing system wherein the polishing operation does not require a prior art conventional MR fluid delivery system.
It is a principal object of the present invention to simplify an MR finishing system to reduce system construction and operating costs, increase percent runtime, improve quality of finished substrates, and increase system flexibility.